Periodontitis is a widespread and costly disease that is primarily manifest in the oral cavity but is also associated with systemic dieases such as atherosclerosis and rheumatoid arthritis. Although several organisms have been identified as periodontal pathogens, A. actinomycetemcomitans has been strongly associated with aggressive forms of periodontitis. Our prior work showed that quorum sensing is essential for A. actinomycetemcomitans virulence and we identified the QseBC two component system as a critical component of the quorum sensing pathway. Our new results suggest that QseBC may interact with catecholamine hormones produced by the host and thus, QseBC may mediate both cell density dependent signaling and inter-kingdom signaling. This application focuses on the mechanisms of interaction of the two component system with catecholamine hormones and is potential role in regulating iron uptake by A. actinomycetemcomitans. Aim 1 will determine if catecholamine hormones and iron represent signals that activate QseBC and identify functional motifs of the QseC sensor that are required for binding. The genes that are regulated upon activation of QseBC will also be identifed using genomic microarrays. Aim 2 will study an operon encoding a putative enterobactin receptor and transporter and determine if this receptor interacts with catecholamine-iron complexes. The contribution of the enterobactin receptor to virulence will also be examined using a mouse model of periodontitis. We have also shown that QseBC may be coupled to the cell stress response through the activity of two iron responsive toxin-antitoxin (TA) systems. This regulatory network will be studied in Aim 3 to determine if the expression of stress proteases is induced under iron limiting conditions and whether these proteases activate the TA toxins by degrading the labile antitoxin. The effect of TA toxin activation on the expression of QseBC and virulence of A. actinomycetemcomitans will also be determined. The overall goal of this study is to better understand the functional outcomes of activation of the essential QseBC two component system and how signaling through QseBC contributes to virulence.